The image resolution that can be achieved from conventional infrared focal plane arrays (IR FPA), even with the most favorable optics, are generally limited by the pixel pitch. In the most advanced conventional FPAs, the smallest pixel pitch dimensions are 12 microns for mid wavelength (MW) IR with a nominal cutoff wavelength of 5 microns. In the long wavelength (LW) IR bands with cutoff wavelengths of around 10 microns, the smallest pixel pitch observed by the applicant for conventional FPAs is 15 microns.
To enhance and further optimize image resolution, the pixel pitch of an FPA for an imaging device or photodetector needs to be comparable to the wavelength of radiation being detected. Among the primary limitations in reducing the pixel pitch of conventional imaging devices or photodetectors are the architecture of the FPA and associated readout circuit contacts and the fabrication technique for electrical interconnection of the FPA to the unit cells of the associated readout circuit. These limitations are especially compelling for the shorter wavelength IR bands with cutoff wavelengths of 2.5 microns or less.
Using high density vertically integrated photodiode (HDVIP®, a trademark of DRS Technologies, Inc.) architecture, a pixel pitch as small as 6 microns and via diameters as small as 2 microns are practically feasible. With these dimensions, fill factors of approximately 90% may be realized. FIG. 1 shows the variation in fill factor versus pixel pitch for an FPA implementing pixels with HDVIP® architecture and having one of three different values of via diameters. As shown in FIG. 1, for smaller pixel pitches (e.g., less than 6 microns), the fill factor drops rapidly even for a via diameter of 2 microns and, thus, compromises the overall photodetector performance.
There is therefore a need for an FPA with associated readout circuit contact architectures and fabrication techniques that enables the realization of imaging devices with pixel pitches approaching the wavelength of radiation to be detected without compromising the fill factor of each photodetector. This is especially a stressing requirement for the short wavelength or SWIR spectral band with cutoff wavelengths ≦2.5 microns. In addition to image resolution, smaller pixel pitch IR FPAs enable reduced size of optics, reduced cooling requirements, which in turn leads to a smaller package, lower power consumption and reduced overall weight.